This invention relates to optical correlators based on charge carrier modulation by optical interference.
Historically, investigation of interaction of light with periodically modulated features in materials dates back to the investigation of ultrasound-induced light diffraction experiments. P. Debye, F. W. Sears, Proc. Nat. Acad. Sci. vol. 18, p. 409, 1932; R. Lucas, P. Biquard, J. Phys. Rad., vol. 3, p. 464, 1932.
Initially, experiments were aimed at investigating such characteristics of ultrasound as velocity of propagation, dispersion, attenuations, reflection, etc. The fortuitous closeness of high frequency acoustical wavelengths in dense materials to the wavelengths of light made these studies successful. Conversely, the understanding of these interactions produced many applications now used in laser engineering. As strong laser sources became available, light alone could produce periodic features in materials which could mimick the standing waves of ultrasound and, therefore, exhibit properties analogous to those observable in acoustooptic interactions. With substantial differences in the dynamic character of the interacting mechanisms, especially on picosecond and femtosecond time scales, the subject of interference-induced material property modulation received considerable attention and produced a significant number of results.
Numerous studies report on both the formation of light-induced spatial modulation in materials and the application of these effects to the study of material properties. In these cases, the effect of optical interference produces periodic changes in the optical parameters which can be attributed to index of refraction modulation, often describable by the third-order nonlinear susceptibility coefficient. N. Bloembergen, et al., IEEE J. QE, vol. 3, p. 197, 1967. W. Kaiser, M. Maier, "Stimulated Rayleigh, Brillouin and Raman spectroscopy," Laser Handbook, vol. 2, ed. by F. T. Arechi, E. O. Schulz-Dubois, Amsterdam: North-Holland, 1972. I. P. Batra, R. H. Enns, D. W. Pohl, Phys. Status Solidi (b), vol. 48, p. 11, 1971. N. Bloembergen, Nonlinear Optics. New York: Benjamin, 1977; S. A. Akhmanov, N. I. Koroteev, "Nonlinear optical techniques in spectroscopy of light scattering," Series Problems in Modern Physics. Moscow: Nauka, 1981 (in Russian); Y. R. Shen, The Principles of Nonlinear Optics. New York: Wiley, 1984. B. Jensen, "Quantum theory of the complex dielectric constant of free carriers in polar semiconductors," IEEE J. Quantum Electron., vol. QE-18, pp. 1361-1370, September 1982.
All of the above cited references study or apply the effects of interference induced diffraction of probe beams. In spite of this effort, there is a cotinually increasing need for optoelectronic devices that are capable of processing ultrashort optical signals and that are suitable for integrated circuit applications.
D. Ritter, et al. have published a paper which discusses the use of two interfering optical beams to measure the ambipolar diffusion length of a photoconductor. D. Ritter, et al.. Appl. Phys. Lett, Vol. 49, No. 13, pp. 791-793, Sept. 29, 1986. In this paper the two interfering beams are of differing intensities, with one much less intense than the other, and the two beams are directed onto the photoconductor to form an interference pattern. Because of the selected beam intensities, the spatial modulation of charge carriers in the photoconductor resulting from optical interference between the two beams is small. The photocurrent varies as a function of the presence or absence of optical interference between the two beams if the ambipolar diffusion length of the charge carriers is sufficiently small with respect to the nodal spacing of the interference pattern. By varying the nodal spacing, the photocurrent can be analyzed to determine the ambipolar diffusion length. The Ritter, et al. article discusses the use of this technique to measure the ambipolar diffusion length of hydrogenated amorphous silicon.
The problem addressed by Ritter, et al. is the measurement of a material parameter of a semiconductor. To this end Ritter, et al. require that the two interfering optical beams be widely different in intensity. Furthermore, the specific material used by Ritter, et al. (hydrogenated amorphous silicon) typically has an electron mobility less than 10 cm.sup.2 /volt-sec.
The present invention is directed to the fundamentally different problem of creating a correlator useful in measuring a selected parameter of one of the two interfering beams (such as amplitude distribution, frequency distribution, or pattern of amplitude modulation, for example). For this reason there are many differences between the structure and operation of the correlators of this invention and the experiments described by Ritter, et al. These differences will be brought out in the following sections.